If you are using SMS Gateway version less than 9.4.x then it can happen because older versions uses Legacy Firebase API which is discontinued by Google. Please upgrade SMS Gateway to latest version to resolve this issue.
You can upgrade to latest version by downloading latest zip file from your CodeCanyon account. Just follow upgrade instructions inside it to upgrade your web app and android app.
If after the upgrade, you can't log in to the web app, then just clear your browser cookies and it should work.